The amount of time you sleep, which includes daytime naps, is linked to your risk of developing cardiovascular disease and early morbidity, according to a study of over 116,000 people in seven regions of the world, published in the peer-reviewed European Heart Journal on Wednesday.

This conclusion appeared to contradict a slew of studies suggesting people should always get more, not less, sleep. They found that people who slept for longer than the usual 6 to 8 hours a day had an increased risk of dying or developing diseases of the heart or blood vessels in the brain.

People who slept for longer than the usual 6 to 8 hours a day had an increased risk of cardiovascular disease and early morbidity, the study found.

Compared to those who slept for the recommended time, people who slept 8 to 9 hours a day had a 5% increased risk of those ailments and early death. while those sleeping between 9 and 10 hours a day had an increased risk of 17% and people sleeping 10 hours a day or more had a 41% higher risk.

“The optimal duration of estimated sleep is 6 to 8 hours per day for adults,” said Chuangshi Wang, a PhD student at McMaster University in Canada and Peking Union Medical College, Chinese Academy of Medical Sciences, China. He cautioned that the results show correlation, not causation.

This study looked at 116,632 adults aged between 35 and 70 years in 21 countries with different income levels in 7 geographic regions (North America and Europe, South America, the Middle East, South Asia, Southeast Asia, China and Africa). They were part a study that started in 2003.

European Health Journal

During an average follow-up time of nearly eight years, 4,381 people died and 4,365 suffered a major cardiovascular problem such as a heart attack or stroke. The researchers adjusted for age, sex, education, smoking, alcohol consumption, lifestyles and their family health history.

Some good news from the study: Daytime napping helped people who slept under 6 hours per night catch up on their sleep and lower the risks of death or cardiovascular problems associated with not getting enough sleep. (The researchers, however, did not collect data on insomnia and apnoea.)

People who are prone to depression and anxiety can also suffer from disturbed sleep and end up oversleeping, which exacerbates those symptoms.

Research published in August suggests people should get between 6 and 8 hours sleep. Epameinondas Fountas, cardiology specialty registrar at the Onassis Cardiac Surgery Centre in Athens, Greece, said that’s a “sweet spot” and the most beneficial for a healthy heart.

President Donald Trump doesn’t get much more than 40 winks. The president gets 4 to 5 hours sleep a night. The late British Prime Minister Margaret Thatcher famously claimed to get 4 hours per night, bolstering her image as “The Iron Lady.” Some historians have cast doubt on that.

However, Fountas also said too much sleep can be detrimental, as well as too little. “We spend one-third of our lives sleeping, yet we know little about the impact of this biological need on the cardiovascular system,” he said. He studied the relationship between sleep duration and cardiovascular disease.

This “meta-analysis” released last July included 11 prospective studies of more than 1 million adults without cardiovascular disease published within the last five years. The research found that both short and long sleepers had a greater risk of developing or dying from coronary artery disease or stroke.

“The relationship between sleep and mood is complex, because disrupted sleep can lead to emotional changes, clinical depression or anxiety, but these conditions can also compound or further disrupt sleep,” according to the National Sleep Foundation. In other words, is the reason for oversleeping the reason for underlying health problems?

Not getting enough sleep is linked with many chronic diseases and conditions — such as type 2 diabetes, heart disease, obesity, and depression.

So people who are prone to depression and anxiety can also suffer from disturbed sleep and end up oversleeping, which exacerbates those symptoms. “If you find yourself sleeping too little or too much on a regular basis, it’s important to bring this up with your doctor so the two of you can look at your total physical and mental health picture,” the foundation says.

Adding to those health risks: “An individual that sleeps on average less than 6 hours per night has a 10% higher mortality risk than someone sleeping between 7 and 9 hours,” according to separate research by RAND Health Quarterly, an online journal. “An individual sleeping between 6 to 7 hours per day still has a 4% cent higher mortality risk.”

Such are the various health dangers associated with a lack of sleep that the CDC describes it a “public health epidemic” in the U.S. Those experiencing a prolonged lack of sleep are also more likely to suffer from hypertension, diabetes, depression, and obesity, and cancer, increased mortality.

Get a daily roundup of the top reads in personal finance delivered to your inbox. Subscribe to MarketWatch's free Personal Finance Daily newsletter. Sign up here.

Intraday Data provided by SIX Financial Information and subject to terms of use. Historical and current end-of-day data provided by SIX Financial Information. All quotes are in local exchange time. Real-time last sale data for U.S. stock quotes reflect trades reported through Nasdaq only. Intraday data delayed at least 15 minutes or per exchange requirements.